Woman injured as rock thrown from Western Freeway off-ramp

Police are searching for a person who threw a rock through a car window in Ballarat, west of Melbourne, injuring a passenger.

The 39-year-old woman was treated at the scene for minor facial injuries after the rock smashed through the car's front passenger window.

Police believe the rock was thrown from an off-ramp on the Western Freeway around 6:30pm.

No-one has been arrested over the incident.

Sergeant Kori Fraser has urged any witnesses to come forward.

"If they've seen any person in that area, no matter how insignificant they think it might be, whether they saw people in a vehicle parked on the side of the road or people walking or on pushbikes, if they could give that information to us that might lead to us identifying these people," he said.